NTSB Identification: SEA07FA014.
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Accident occurred Thursday, November 02, 2006 in Kalispell, MT
Probable Cause Approval Date: 11/29/2007
Aircraft: Bell 407, registration: N407KH
Injuries: 3 Uninjured.
NTSB investigators either traveled in support of this investigation or conducted a significant amount of investigative work without any travel, and used data obtained from various sources to prepare this aircraft accident report.
While conducting EMS operations the helicopter sustained substantial damage after a loss of engine power, autorotation, and forced landing. The pilot reported hearing a loud bang, followed by the engine spooling down. The helicopter collided with a chain link fence during the forced landing sequence. Postaccident inspection of the engine revealed an uncontained second stage turbine wheel burst (on-speed) as a result of oil starvation and subsequent bearing failure. During the onsite examination, fresh oil residue was noted along the right side of the helicopter's fuselage, tail boom assembly and leading edges of the tail rotor blades. Oil wetting and staining was also observed on the concrete hangar apron, and a trail of fresh oil extended from apron to the helipad where the helicopter departed. Additionally, onsite investigators noted that the oil pressure transducer line coupling B-nut (near the foreword bulkhead) was found two flats loose, and four complete rotations from complete separation. Evidence of oil loss, sooting and localized fire damage was observed in the area of the fitting union. The accident flight was the helicopter's first flight following the completion of a maintenance action item in the general area of the aforementioned fitting. Following the maintenance action, an engine run (ground) and post run inspection was completed by a line pilot. No discrepancies were noted and the helicopter was returned to service. The mechanic who performed the maintenance action was not present during the post run inspection.
The National Transportation Safety Board determines the probable cause(s) of this accident to be: On-speed failure of the second stage turbine wheel as the result of engine oil starvation. The lack of maintenance oversight was a contributing factor. Full narrative available
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